refactor(exporter-metrics-otlp-http): fix eslint warning

Fixes the following eslint warning in exporter-metrics-otlp-http:

```
/home/runner/work/opentelemetry-js/opentelemetry-js/experimental/packages/opentelemetry-exporter-metrics-otlp-http/src/OTLPMetricExporterBase.ts
  120:30  warning  Unexpected any. Specify a different type  @typescript-eslint/no-explicit-any
```

The unused parameter can be explicitly typed as `InstrumentType`,
but since this default selector doesn't actually care about it, it
can also just be elided.

By adding an explict return type, if someone in the future added
the wrong parameter to the default selector function, TypeScript
would reject it.

Ref open-telemetry#5365

experimental/packages/opentelemetry-exporter-metrics-otlp-http/src/OTLPMetricExporterBase.ts

@@ -111,18 +111,14 @@ function chooseTemporalitySelector(
   return chooseTemporalitySelectorFromEnvironment();
 }
 
+const DEFAULT_AGGREGATION = Object.freeze({
+  type: AggregationType.DEFAULT,
+});
+
 function chooseAggregationSelector(
   config: OTLPMetricExporterOptions | undefined
-) {
-  if (config?.aggregationPreference) {
-    return config.aggregationPreference;
-  } else {
-    return (_instrumentType: any) => {
-      return {
-        type: AggregationType.DEFAULT,
-      };
-    };
-  }
+): AggregationSelector {
+  return config?.aggregationPreference ?? (() => DEFAULT_AGGREGATION);
 }
 
 export class OTLPMetricExporterBase